A study published yesterday outlines a new way to map the emissions produced by cities. Cities are major contributors to global climate change, so the authors say this could be an important step forward in meeting emission reduction targets.
Cities — as hubs of industry, housing, business and transport — are responsible for more than 80 per cent of greenhouse gas emissions worldwide, according to the World Bank. With the global population approaching nine billion and 68 per cent of people expected to be living in urbanised areas by 2050, the way cities develop in the future is set to be critical when it comes to setting and meeting international emission reduction targets.
While all countries under the United Nations Framework Convention on Climate Change (UNFCCC) are legally obliged to report their national greenhouse gas emissions, different countries use different methods to do so. This inconsistency filters through to the city level. Although a large chunk of the responsibility to reduce national emissions will fall to cities, an international standard for working out how much cities contribute also isn’t settled.
A new study published yesterday in the journal Environmental Science and Technology lays out a new method for quantifying greenhouse gas emissions for individual buildings every hour for an entire city. Professor Kevin Gurney from the Global Institute of Sustainability at Arizona State University and lead author of the study explains why this is important:
“Cities have had little information with which to guide reductions in greenhouse gas emissions — and you can’t reduce what you can’t measure.”
Tracking emissions Previous research has tended to estimate cities’ emissions on a much broader level, based on data on industrial energy use and average domestic consumption. The new research takes advantage of the growing number of atmospheric measurements made at city level to dig deeper into exactly where the emissions come from.
A still from a video of hour-by-hour changes in carbon dioxide emissions from different building types for the US city of Indianapolis. Credit: Bedrich Benes and Michel Abdul-Massih
The team of scientists collected a range of ‘bottom up’ data about carbon dioxide emissions from air pollution reports, traffic surveys and basic information about building type and size collected for tax purposes. They combined these data with a computer modelling system which calculated energy consumption on a building-by-building basis.
As Gurney explains, users can track the emission intensity for the whole city using colour-coded, high resolution maps and use them to make decisions on where efforts to reduce emissions would be most effective. He says:
“…[T]he emission mitigation impact of targeting older, less insulated homes can be quantified and compared to targeting zones of congested, high-emitting on-road sources.”
As well as providing information in a way that is useful to local decision-makers, the colourful visual displays of daily or seasonal variations in emissions from different sources — residential buildings, industry and transport — also make it easier for ordinary people to see the impact they can make locally to help mitigate climate change. Gurney adds:
“…[W]e can provide cities with a complete, three-dimensional picture of where, when and how carbon dioxide emissions are occurring…to identify the most cost-effective ways to reduce emissions and track progress over time.”
The issue of scale is important in emissions accounting for cities. Previous research for the city of Toronto has shown that the differences in emissions intensity within the city can be as big as the difference between cities. For example, residents of high-density apartment blocks in dense city centres can emit half as much carbon dioxide as neighbouring residents in single family homes in the suburbs.
So far the researchers have used the new method in one city in the United States, Indianapolis, but they say it can be reproduced across all major US cities, which together are responsible for nearly 25 per cent of all global carbon dioxide emissions. The researchers are already working on estimates for Los Angeles, California and Phoenix.
What about international treaties?
On an international scale, the team suggests this new tool may help encourage the US to sign up to international climate change treaties. In the past, comparing different countries’ stated emissions without having a way to independently verify them has cast doubt over how to incorporate them into mitigation agreements to follow on from the Kyoto Protocol.
The study is part of a larger project called ‘Hestia’, named after the Greek goddess of the hearth and home, which combines ground-based and satellite measurements of methane and carbon monoxide as well as carbon dioxide. The results from Hestia are expected to complement the launch of the Orbital Carbon Observatory satellite, planned for December 2013, which will measure the concentration of carbon dioxide in the atmosphere.
One thing to note is that the new method does not account for material or electricity consumed within the city but produced elsewhere. A study led by the World Bank in 2011 shows that the greenhouse gas emissions for the same urban resident can vary significantly depending on whether production or consumption, or both, are taken into account when calculating greenhouse gas emissions.
All this shows that clarity counts when calculating urban emissions if countries are to adopt an international standard. But this research by US scientists could make a valuable contribution to creating a standard that countries can agree on.
The full video for the Hestia projects and shorter videos of emission calculations over time for Indianapolis can be found on the project website here.
This piece was originally published at The Carbon Brief and was reprinted with permission.